Oils and other fluids are used in various applications, including, for example, lubrication of machinery and to apply hydraulic force to various actuators. Such systems may be substantially closed, as in an engine lubrication application, or open, such as in a hydraulic system having a vented tank. In both those systems, the fluids may be replaced frequently. Such replacement may not be required because the fluid itself is ineffective, but rather because the fluid has become suffused with undesirable materials, such as particulates, water, or uncombusted fuel. Thus, it is believed that there is a need for filtration systems and methods that improve the cleanliness of such fluids to save the costs of labor and replacement fluids. It is furthermore believed that there is a need for filtration systems and methods that improve the cleanliness of such fluids to minimize wasting resources, such as oil and hydraulic fluid, which may continue to be effective once cleaned.